This invention relates generally to an apparatus and method for certifying a coordinate measuring machine (CMM) and, more particularly, to a method and apparatus for measuring the repeatability and volumetric accuracy of a coordinate measuring machine. The invention is useful for certifying both manually operated, user manipulable, as well as programmable coordinate measuring machines.
Coordinate measuring machines are capable of accurately gauging, in three-dimensional space, the surfaces of a machined part, or the like, in order to determine compliance with manufacturing tolerances for the part. In order to verify the dependability of the results obtained with the CMM, various techniques have been developed for certifying the performance of the CMM.
One such method and apparatus for certifying a CMM is the subject of an industry standard: ANSI/ASME B89.1.12M-1985. This standard prescribes two independent gauges in order to determine the repeatability and accuracy of the CMM. The first gauge is a bar with balls affixed at each end, known as a "ballbar." While the diameters of tile balls are certified to a known diameter, the distance between the balls is not known with accuracy. Such balls are threaded on and off of the bar for storage purposes, which precludes the ability to certify the center-to-center distance between the balls. The ballbar is positioned in various generally determined locations within the three-dimensional measuring volume of the CMM. The CMM probe and stylus are used to measure the ball diameters. The CMM calculates the diameter of each ball and the center-to-center spacing between the balls. This process is repeated with each value of ball diameter and center-to-center spacing being compared with the other values in order to determine the repeatability of the CMM. In order to certify the linear accuracy of the CMM, the ANSI/ASME standard prescribes a second gauge, namely a step block, which is repositioned along each of the three axes of the CMM. Measurements are made at various of the steps in order to determine the linear accuracy of the CMM scales.
While the ANSI/ASME standard is accepted in the industry and provides a reasonably accurate determination of the important parameters for certifying a CMM, one significant limitation is its difficulty to use. It requires the use of two separate gauges with separate measurement procedures in order to measure the volumetric accuracy and repeatability of the CMM. Another major limitation is that the ANSI/ASME ballbar/step block standard has been useful only in certifying manual operator manipulated CMMs. The ANSI/ASME standard has not been practical for use with programmable CMMs. The difficultly with applying the ANSI/ASME standard to programmable CMMs lies in the manner in which programmable CMMs operate. A programmable CMM must be provided with the location of each point to be measured within approximately 50 thousandths of an inch (0.050 inch). The CMM rapidly moves the stylus to the given location and more slowly moves the stylus until a surface is contacted. A touch-trigger probe supporting the stylus senses the contact and records the position of the stylus when contact is made. The reason the ANSI/ASME standard has not proven practical with programmable CMMs is that the positioning of the ballbars is not sufficiently accurate to allow the programmable CMM to be programmed to measure points on the surfaces of the spheres of the ballbar and, therefore, the automatic movement feature of such programmable CMM cannot be used with the ballbars.
One prior art attempt at overcoming the limitations of the ANSI/ASME standard includes a bar that is universally pivotally supported at one end by a pivot ball for 360.degree. movement in a horizontal plane and approximately .+-.45.degree. movement in a vertical direction. Guide means, including a pair of parallel rods and a spherical abutment between the rods, is provided at the other end of the bar. A special, certified stylus is fitted to the CMM probe. In use, the certification device is positioned within the measurement volume of the CMM. The bar is removed and the CMM probe is used to establish the location of the pivot ball of the universal pivot in the CMM coordinate system. The bar is replaced and allowed to thermally stabilize and the stylus is manually positioned within the guide means of the bar. The CMM is programmed to rotate the bar around the universal pivot while making measurements at various locations along the imaginary sphere traced by the movement of the guide means. The "length" of the bar between the guide means and pivot is determined at various points in the sphere. Differences in length measurements are used to calculate straightness, squareness and axis roll.
While this approach overcomes the limitations of the ANSI/ASME standard, it is not without its own limitations. The device does not provide satisfactory measurements of the volumetric accuracy of the CMM. Although it measures the distance between the contact point within the guide means and the point of pivot at the universal pivot, the movement between the stylus and the guide means, and the movement of the bar about the universal pivot, precludes a meaningful comparison being made between a measured distance and a sufficiently accurate certified distance as would be required to determine volumetric accuracy. Thermal instability inaccuracies, resulting from manual handling of the bar, only contribute to the problem. Another major deficiency is that it is difficult to use with a manual CMM. The bar is balanced with a counterweight to have a slight downward bias at the guide means. The operator is typically not able to control the interface between the stylus and the guide means sufficiently in order to avoid frequent disengagement with the guide means. When the stylus disengages from the guide means, the downward bias of the guide means causes it to abruptly drop, resulting in the necessity for re-establishing contact between the stylus and the guide means. Furthermore, under any circumstances, it would be impractical for this device to certify the articulation capabilities of the motorized head used with many programmable CMMs to reorient the probe.
A need exists for a coordinate measuring machine certification apparatus and method that is useable with both user controlled manual CMMs and programmable CMMs. The need exists for a CMM certification method and apparatus that is capable of accurately certifying the repeatability and the volumetric accuracy of a CMM in a single series of measurements.